Yet again I was inspired to write about today’s topic, AC Supercooling based on people’s questions on Facebook. Technically, supercooling has nothing to do with air conditioning – the industry “borrowed” the word, more precisely, utility companies borrowed it.
I also posted a video about AC supercooling on the Magic Touch Mechanical YouTube channel if you prefer videos.
What is AC Supercooling?
As it’s a borrowed word, I’m not sure if it should be AC supercooling or AC super cooling. AC super-cooling seems more accurate to me since we’re discussing cooling your home with your air conditioner. I do whatever I want, so I’ll use all of them interchangeably in this article (like a rebel)!
So, what is AC super cooling? Super cooling is setting your AC thermostat to a very low setting when your utility company’s rates are the lowest. The idea being you basically “freeze” out your home during the hours when the utility rates are at their lowest. Then, during peak rate hours (highest rate), shut off your AC unit and allow the temperature to slowly “drift” up. In a perfect world, your home’s temperature would slowly climb, and you run your AC again after peak hours end.
Does AC Super Cooling Your Home Work?
AC Super cooling your home does work in some homes, however it does not work in all homes. There are many variables to consider with regards to the home’s design, the home’s efficiency, your rate plan, and more.
Other considerations include, lifestyle, who’s home when, pets (especially reptiles, birds, and small mammals), children, and the elderly. Remember you’re basically “freezing out” the place … including everyone and everything inside. In other words, everyone is going to be very cold for a few hours, and then get warmer and warmer. You’ll be sacrificing comfort for energy savings.
Another key factor is the type of air conditioning units your home has. AC super-cooling was originally intended to work for homes with single-stage AC units.
Here in Phoenix, two-stage, and variable speed (inverter) AC units are becoming more prevalent (approximately 90% of our new installations). This technology was designed to run constantly at lower speeds (consuming less energy), so supercooling forces them to run inefficiently.
When you supercool with a variable speed AC, you’re forcing it to run at 100% during super cooling. Then again when you first turn it back on, it must quickly correct wherever the temperature “drifted” to – likely at 100%. In other words, you’re wasting all that high-efficiency technology by forcing it to run like a single-stage AC unit.
Case Studies Demonstrating When It Works Well
Having personally conducted thousands of home energy audits throughout Phoenix, I can attest supercooling does indeed work … in some homes. I have clients who have dropped their summer cooling bills significantly using the AC supper cooling method.
Those with the greatest success live in homes with a very tight building envelope. Meaning, very little air infiltration (air leakage into the home). These homes are also very efficient, i.e., well insulated, window sunscreens, light colored roofs, well shaded, etc.
In a very efficient home, the temperature “drift” upwards after turning off the AC is slow. In other words, that home retains that cooled air much longer than a less efficient home would.
Why AC Supercooling Does Not Work in All Homes
Modern building practices have increased the number of homes being built that by design are relatively efficient. Some builders even specialize in building high-efficiency homes.
However, most homes across the USA are tract homes built to pass the minimum efficiency codes to keep costs low. The “money” is spent to offer nice countertops, flooring, cabinets, and fixtures, etc., rather than improving efficiency. Why, because pretty kitchens & baths sell houses, not more insulation, high-efficiency AC units, or tighter building envelopes.
In most tract homes, the temperature “drift” is rapid so the home can’t contain that cold air for very long. AC super cooling doesn’t work well in these types of conditions for that reason.
Here in Phoenix, we also have many homes that were built in the 1980’s or prior. Most of these homes were built without much thought for energy conservation of efficiency. In fact, older homes built in the 50’s, 60’s, and even 70’s often used swamp coolers instead of central air. These homes were not well insulated at all and had air leaks like water moving through a spaghetti strainer. Even after home improvements have been made, some of these homes are still not good candidates for AC super cooling.
Super Cooling Works Best with Specific AC Units
Let’s face it, if you’re considering AC supercooling your home it’s because you want to lower your electricity bill. Believe it or not, AC supercooling works out best for people who have lower-efficiency, single-stage air conditioners.
Why? When you drop the temperature dramatically on very hot days, you’re telling the AC unit to run at full capacity. Basically, you’re telling your AC unit to run “full blast”. Because single-stage air conditioners only have one speed (100%), they’re technically always running full-blast anyway. One speed AC units are also the least efficient type of unit. By running them when utility rates are their lowest and shutting them off when rates are the highest (super cooling), you’re going to save money. Again, provided your home is efficient enough to hold off the temperature drift upwards for the duration of peak rate hours.
In these extreme temperatures, some two-stage AC unit owners may also see some savings. Two-stage units operate in low & high so while more efficient than 1-speed, they’re not as efficient as variable speed units.
Variable Speed Units A.K.A. Inverter AC Units
My home is equipped with three variable speed AC units. Two of my units condition our main home and one conditions my workshop (a converted garage) that’s been insulated. I do not supercool my home or workshop with my air conditioners. Although, when people discover I cool my home 24/7 to 69°-70°F, they might say otherwise … we like it cold!
That said, I’m able to cool my home to that temperature for a few reasons.
- I practice what I preach. My home is extremely efficient because it’s well insulated, sealed, and I have high-quality sunscreens on all windows.
- Since all my units are variable-speed and I run a consistent temperature, they rarely need to run at 100%. In fact, they tend to run at a very low speed consistently.
- I choose SRP’s one-rate plan, meaning I pay the same rate regardless of the time or day. That plan is right for my home and our lifestyle.
The result is low utility bills without sacrificing comfort. There was an upfront cost to achieve these results, but we value comfort, so for us it was worth it. Having been in the HVAC business for over 35-years, I’ve spent my fair share of time in the sun and in a hot attic. I want to be comfortable at home all the time! I also don’t want to spend a fortune to keep my home that cool so I made my home and HVAC as efficient as possible.
Bottom line, if you have variable speed AC units, you’ll probably not save much (if anything) by AC supercooling. Let your machines run as designed – constantly at lower speeds. Of course, you still need to be sure your home itself is efficient, if not, here’s how to do it.
Make SURE You’re on The Right Utility Rate Plan
AC super-cooling on the wrong utility rate plan could wind up costing you a fortune. I’ve seen it before. A homeowner hears about super cooling, tries it for a month and gets an unpleasant surprise – an outrageous power bill!
Remember AC super cooling is meant to reduce power consumption when rates are their highest.
Take my plan, the straight rate plan. I pay a lower rate during peak hours and a higher rate during off-peak hours – the rate never changes. Plans like the ‘time of use” plan have one rate on peak, one rate off peak. Other plans have multiple rates throughout the day and on different days of the week. Bottom line, make sure you understand what you’re doing and why before trying supercooling with your air conditioner.
Comfort vs Energy Savings
We all prioritize and value what’s important to us. Some people are willing to sacrifice comfort for lower power bills. Others, prioritize their comfort no matter the cost. Personally, I’m in the prioritizing comfort group, but with a twist – I spent more upfront to have my cake and eat it too.
I may recover the money I spent upfront over the lifetime of my equipment and time I own this home. I may not – but it will be close. If you’re like me, consider the home improvements needed to have both comfort and low monthly bills. If you’re in the other group, supercool … just make sure your home & rate plan are appropriate for it!
